Japanese scientists predict that radioactive material leaking from the crippled Fukushima nuclear plant will spread 4,000 kilometres through the Pacific Ocean in the next year.
Japan’s Atomic Energy Agency says the caesium contamination will also reach Hawaii in three years,
Then it will wash along the United States west coast in five years.
But the agency says by that time the caesium will not pose a threat to human health.
Caesium, with its isotope caesium-137, is formed as a product of nuclear fission.
Caesium-137 has a half-life of about 30.17 years.
It is quite chemically reactive and difficult to handle. Caesium salts are very soluble in water and this complicates its safe handling.
Meanwhile, a leak at the nuclear plant has forced workers to halt the pumping of decontaminated run-off water being used to cool reactors.
A spokesman from the operator, TEPCO, says engineers will check the system today.
It connects reactors with water purification facilities.
The hope is to restart it as soon as possible.